CODEWITHSUNDEEP
node.jsexpressasynchronous
Fuller
Fuller

Reputation: 301

Accessing data from multiple callbacks

I am developing an Express nodeJS web app and am trying to increase my knowledge of asynchronous programming and how to access the data that I need properly.

I have two callback functions which query an SQL database. My functions are declared like so:

function CustomerList(callback) {
  const db = require('../db');
  db.query('SELECT * FROM users WHERE role = (?)', ['User'], (error, results, fields) => {
    if (error) throw error;
    callback(results);

});

}

function AdminList(callback) {
   const db = require('../db');
   db.query('SELECT * FROM admins WHERE role = (?)', ['Moderator'], (error, results, fields) => {
    if (error) throw error;
    callback(results);

});

}

I use the data retrieved from both callbacks and send it to my view engine template to be displayed in the browser like so:

 CustomerList((userData) => {  

      AdminList((adminData) => {

          res.render('adminhome', {
              title: 'Admin - Home',
              results: 'test',
              customers: userData,
              admins: adminData
          });

      });

This works as expected and I can manipulate the data however I please on my view template. But... This seems... 'clunky', and I feel like this method I am using is going to cause me trouble in the future if I create additional functions to retrieve more data, and then keep nesting additional callbacks to pass the data to my view template.

Essentially, I am struggling to find a suitable approach to retrieve the UserData results and AdminData results and pass them to my res.render API call.

What are better alternatives to my current approach?

Upvotes: 0

Views: 198

Answers (1)

Rahul Sharma
Rahul Sharma

Reputation: 10071

Yes, it'll create callback hell if you call multiple callbacks inside the callback, use promises(async/await)

const util = require('util');
const db = require('../db');
const query = util.promisify(db.query);

const getData = async () => {
    try {
        const CustomerList = await query('SELECT * FROM users WHERE role = (?)', ['User']);
        const AdminList = await query('SELECT * FROM admins WHERE role = (?)', ['Moderator']);

        // OR Promise.all

        const [CustomerList, AdminList] = await Promise.all([
            query('SELECT * FROM users WHERE role = (?)', ['User']),
            query('SELECT * FROM admins WHERE role = (?)', ['Moderator'])
        ]);
    } catch (error) {
        console.log(error);
    }
}

Upvotes: 1

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